1. Field of the Invention
The present invention relates to a multilayered ceramic substrate produced by applying a non-shrinkage process which is capable of substantially preventing the occurrence of shrinkage in the direction parallel to a main surface in a firing step, and a method of producing the same. Particularly, the present invention relates to a multilayered ceramic substrate comprising at least two types of ceramic layers having different characteristics, and a method of producing the same.
2. Description of the Related Art
Some multilayered ceramic substrates have a stricture comprising a built-in passive part such as a capacitor element or an inductor element provided for making the substrates multifunctional. A typical example of the structures of such multifunctional multilayered ceramic substrates comprises a plurality of insulating ceramic layers comprising insulating ceramic materials which constitute a laminated structure in a multilayered ceramic substrate, at least one the insulating ceramic layers being replaced by a dielectric ceramic layer or a magnetic ceramic layer containing a dielectric ceramic material or magnetic ceramic material so that a capacitor element or the inductor element is formed in connection with the dielectric ceramic layer or the magnetic ceramic layer.
In producing the above-described multilayered ceramic substrate, besides an insulating green sheet containing an insulating ceramic material, a dielectric or magnetic green sheet containing a dielectric or magnetic material different from the insulating ceramic material is prepared, and these different types of green sheets containing different ceramic materials are laminated to obtain a laminated structure, followed by co-firing.
In the firing step, the different ceramic materials contained in the respective green sheets generally exhibit different behaviors of sintering shrinkage, and thus in order to permit co-firing of the different types of green sheets, it is necessary to cause the sintering shrinkage behaviors to coincide with each other as much as possible.
Therefore, a measure is taken in which the ceramic material contained in at least one type of green sheet is changed, glass is added to at least one type of green sheet or a portion of the ceramic material contained in one type of green sheet is added to the other type of green sheet.
However, the above measure to change the ceramic material, add glass or add a portion of the ceramic material contained in one type of green sheet to the other type causes an undesired change or deterioration in various characteristics, for example, such as insulation resistance, dielectric constant, dielectric loss, temperature characteristics, etc. in some cases.
The above-described addition causes a change in characteristics in the interface between ceramic layers due to mutual diffusion of the added glass or ceramic material, thereby causing dispersion of characteristics.
In order to solve the problem of mutual diffusion, an attempt is made to provide a diffusion inhibiting layer as an intermediate layer between different types of green sheets to inhibit the diffusion or lengthen the diffusion length, thereby decreasing the effect of the mutual diffusion.
However, the adoption of the above measure requires the coincidence of the shrinkage behavior of the intermediate layer serving as the diffusion preventing layer with the other green sheets, thereby causing the need to add glass or the like to the intermediate layer. Therefore, the above measure can obtain only the effect of relieving a change or deterioration in characteristics with a gradient between different types of ceramic layers.
With a great difference between the shrinkage behaviors of different types of adjacent green sheets, the thickness of the intermediate layer must be significantly increased, or a plurality of intermediate layers having a stepwise thickness gradient are required for relieving the stress caused by the difference between the shrinkage behaviors.
However, an increase in the thickness of the intermediate layer, or the formation of a plurality of intermediate layers leads to deterioration in characteristics of the multilayered ceramic substrate or an increase in the thickness thereof, thereby causing many practical problems in the multilayered ceramic substrate which can be integrally fired.
Accordingly, it is an object of the present invention to provide a multilayered ceramic substrate and a method of producing the same which can solve the above problem.
In order to solve the above technical problem, the present invention provides technical means in which two substrate green sheets containing different low-temperature sintered ceramic materials are fired in a state wherein a shrinkage inhibiting green sheet containing an inorganic material which is not sintered at the sintering temperature of each of the low-temperature sintered ceramic materials is held between the two substrate green sheets. Therefore, the shrinkage inhibiting green sheet restrains the substrate green sheets to prevent the occurrence of sintering shrinkage in the direction parallel to main surfaces so that shrinkage occurs only in the direction of the thickness. Thus, even with a difference between the shrinkage behaviors of the two substrate green sheets, the occurrence of stress due to the difference between the shrinkage behaviors of the two substrate green sheets can be substantially prevented.
By using the technical means, the shrinkage inhibiting green sheet serving as the intermediate layer becomes a layer in which many voids are present in an unsintered state because the inorganic material contained in the shrinkage inhibiting green sheet need not be sintered in the firing step. On the other hand, the low-temperature sintered ceramic materials contained in the substrate green sheets produce fused glass by firing, and the fused glass is absorbed by diffusion into the voids of the inorganic material. Therefore, glass diffusion from one of the substrate green sheets to the other substrate green sheet can be inhibited to prevent the occurrence of a change or deterioration in characteristics.
The intermediate layer has no need to obliquely relieve stress by using the thickness, and a thickness designed to be sufficient for absorbing the glass caused by the substrate green sheets can be used, thereby causing no need to significantly increase the thickness.
By using the technical means, a multilayered ceramic substrate and a method of producing the same are provided.
Namely, the multilayered ceramic substrate of the present invention comprises at least two types of substrate ceramic layers containing different low-temperature sintered ceramic materials, a shrinkage inhibiting layer which is provided between the different types of substrate ceramic layers, and which contains an inorganic material in an unsintered state, which is not sintered at the sintering temperature of each of the low-temperature sintered ceramic materials, so that the inorganic material is fixed by glass permeating into the inorganic material during firing the low-temperature sintered ceramic materials contained in the substrate ceramic layers, and a wiring conductor provided in connection with the substrate ceramic layers and/or the shrinkage inhibiting layer.
In the multilayered ceramic substrate of the present invention, as a conductive material which constitutes the wiring conductor, for example, a low-resistance conductive material comprising, as a main component, at least one metal selected from Ag, Au, Cu, Agxe2x80x94Pd and Agxe2x80x94Pt can be used.
The wiring conductor is formed in the multilayered ceramic substrate or formed on the external surface. The wiring conductor comprises, for example, a planar conductor provided along a main surface of the specified substrate ceramic layer or a via hole conductor provided to pass through a specified substrate ceramic layer and/or the shrinkage inhibiting layer according to the design of electric connection required for the multilayered ceramic substrate.
The multilayered ceramic substrate of the present invention may further comprise a cavity having an opening which faces the outer surface thereof.
The method of producing a multilayered ceramic substrate of the present invention comprising the step of preparing a green composite laminated product comprising at least two types of substrate green sheets containing different low-temperature sintered ceramic materials, a shrinkage inhibiting green sheet which is provided between the different types of substrate green sheets and which contains an inorganic material not sintered at the sintering temperature of each of the low-temperature sintered ceramic materials, and a wiring conductor provided in connection with the substrate green sheets and/or the shrinkage inhibiting green sheet, and the step of firing the green composite laminated structure.
In the firing step, the low-temperature sintered ceramic material contained in each of the substrate green sheets is sintered while inhibiting shrinkage in the direction of the main surfaces of each of the substrate green sheets by the shrinkage inhibiting green sheet, and the inorganic material contained in the shrinkage inhibiting green sheet is fixed by glass permeating into the inorganic material when firing the low-temperature sintered ceramic materials, while being left in an unsintered state.
In the method of producing a multilayered ceramic substrate of the present invention, the step of preparing the green composite laminated structure may comprise the step of preparing each of the substrate green sheets and the shrinkage inhibiting green sheet, and the step of laminating the substrate green sheets and the shrinkage inhibiting green sheet, or comprise the step of preparing the substrate green sheets, the step of preparing a slurry containing the inorganic material, and the step of coating the slurry on each of the substrate green sheets to form the shrinkage inhibiting green sheet. As a modified example of the latter case, in order to prepare the green composite laminated product, the shrinkage inhibiting green sheet may be prepared, and a slurry containing a low-temperature sintered ceramic material may be prepared so that the slurry is coated on the shrinkage inhibiting green sheet to form the substrate green sheet.
In the method of producing a multilayered ceramic substrate of the present invention, the low-temperature sintered ceramic material contained in each of the substrate green sheets is preferably sinterable at a temperature of about 1000xc2x0 C. or less.
In the method of producing a multilayered ceramic substrate of the present invention, the thickness of the shrinkage inhibiting green sheet is preferably selected in the range from about 1 xcexcm to less than about 30 xcexcm.
In the method of producing a multilayered ceramic substrate of the present invention, the shrinkage inhibiting green sheet may contain at least one additive selected from glass, oxides and metals for supporting the fixing of the inorganic material in an amount of about 60% by weight or less based on the inorganic material. In this case, the thickness of the shrinkage inhibiting green sheet can be increased to about 50 xcexcm or less.
In the method of producing a multilayered ceramic substrate of the present invention, the sintering temperature of the inorganic material contained in the shrinkage inhibiting green sheet or the sintering temperature of the inorganic material and the additive for supporting sintering of the inorganic material is preferably about 100xc2x0 C. or more higher than the sintering temperature of each of the low-temperature sintered ceramic materials.